Purification of zinc solutions



UNITED STATES PATENT OFFICE.

SAMUEL FIELD, OF LONDON, ENGLAND, ASSIGNOR TO THE METALS EXTRACTIONCORPORATION LIMITED, OF LONDON, ENGLAN'D.

PURIFICATION OF ZINC SOLUTIONS.

No Drawing.

To all whom it may concern:

Be it known that I, SAMUEL FIELD, a subject of the King of England,residing at London, England, have invented certain new and usefulImprovements in the Purification of Zinc Solutions, of which thefollowing is a specification.

This invention is for improvements in or relating to the preparation ofzinc solutions for electrolysis, and itrefers more particularly to' thepurification of zinc sulfate solutions obtained'by the treatment of zincores or zinc-containing materials with sulfuric acid from whichsolutions the zinc is subsequently deposited electrolytically.

As is well-known, to obtain a successful deposition of zinc byelectrolysis, the .zinc sulfate liquors must be of a high order ofpurity; impurities such as copper, cadmium, iron antimony, arsenic,nickel and cobalt must be completely eliminated or substantially so, asthey affect adversely and very seriously the deposition bothqualitatively and quantitatively.

In some cases, as with cadmium, the metal is deposited with the zinc andrenders it brittle and valueless. In the treatment of zinc sulfatesolutions by electrolysis I have found that one of the most seriousimpurities is the metal nickel, which even in minute quantities,prevents the satisfactory formation of a zinc deposit on the cathode;indeed, after a good deposit of zinc has been obtained, if nickel entersinto the solution, the deposit of zinc may become pitted and rapidlydisappear, and to get a satisfactory electrolysis of zinc sulfatesolutions I have found it necessary that the nickel be reduced to aquantity of the order of one-half to one part per million of liquor orless.

It has hitherto been common practice to remove these impurities in aseries of steps by precipitating each with a suitable preclpitant. It isan object of the present invention to provide a process whereby a numberof metals other than zinc may be completely precipitated in a singleoperation.

Copper, cadmium and similar very elec- Specification of Letters Patent.

Patented Apr. 13, 1920.

Application filed September 26, 1919. Serial No. 326,577.

tro-negative metallic impurities have hitherto been removed from zincsolutions by tube milling with zinc balls or agitating the solutionswith zinc dust, and the methods employed for removing iron also resultin the simultaneous removal of arsenic and antimony when present.

solutions with zinc fume or blue powder in the presence of a smallproportion of'acid, for example 0.1% or less at a temperature of 80 to100 C. whereby the nickel content of the solution is reduced tosubstantially 0.5 part per million or less.

In carrying the present invention into effect to obtain a pure solutionof zinc sulfate from ores or zinc in containing materials for thesubsequent electro-deposition of the zinc, the zinc ore orzinc-containing material, roasted if necessary, is treated with theacid, for example, acid liquors from the electrolytic cells in whichzinc has been deposited, and from the solution'thus obtained any iron,arsenic and antimony are first removed by known methods. The solutionsare run into a tank fitted with suitable arrangements for heating, andprovided with a paddle or agitator to effecif gentle stirring. Sulfuricacid is added in a quantity of 7 to 14 lbs. of 70% acid to 5 to 0 tonsof liquor, and the solution is raised to a temperature of from 80 C. to100 C. The zinc fume or blue powder is then added. A much largerproportion of zinc is employed than is theoretically necessary toreplace chemically or electrochemically the impurities.

The solution is stirred gently in order to bring about a maximum contactbetween the zinc and the liquors, and any copper. cadmium and nickelpresent are readily precipitated. Thus the three metals, copper, cadmiumand nickel are readily removed in a single operation with concomitantsaving of time, labor and other charges, reducing the cost ofpurification.

It has, however, been found that the re moval of nickel and cadmium fromzinc solutions by this method is greatly facilitated by the presence ofcopper. Normally copper is present in many of the ores and productswhich are amenable to the electrolytic recovery of zinc. In the absenceof copper and to take advantage of this discovery I add copper to thesolution by one or other of several methods. Thus a $0111- ble coppersalt, for example copper sulfate, may be added to the solution.Alternatively a small quantity of suitable copper ore, for example, oneof the carbonates may be added in the process of extracting the zincfrom the ore or other zinc-containing material. When copper is presentthe immediate precipitation of this more electronegative impurityestablishes electro-chem- -ical conditions which effect theprecipitation of cadmium and nickel.

It has further been observed that when zinc is used at an elevatedtemperature for the removal of nickel in the absence of acid, there is atendency to the formation of basic zinc sulfate. This substance producedon the zinc slows down the rate of precipitation of the nickel. With theuse of acid as herein described this formation of basic zinc sulfate ismuch less marked. If formed, however, its retarding effect on theprocess of purification can be counteracted by the addition of afurthersmall quantity of acid, or alternatively, the basic sulfated zincmay be washed with weak sulfuric acid which can be subsequently added tothe acid liquor used for the solution of the zinc from the ore or othermaterial. The

acid Washed zinc is thus rendered effective for further purification.

What I claim as my invention and desire to secure by Letters Patentis 1. The hereindescribed process for the preparation of zinc sulfatesolutions (free from copper) for electrolysis by purifying the solutionsfrom nickel and cadmium, which consists in adding a small proportion ofacid, heating the solution to a temperature of 80 to 100 0., adding asmall proportion of copper in soluble form, adding zinc powder, andagitating the mixture whereby the nickel content of the solution isreduced to substantially 0.5 part per million or less.

2. The hereindescribed process for the preparation of zinc sulfatesolutions for electrolysis by purifying the solutions from nickel,copper and cadmium, which consists in adding a small proportion of acid,heating the solution to a temperature of 80 to 100 C., adding zincpowder, agitating the mixture and adding free acid to the zinc solutionduring the process of purification, whereby the nickel content of thesolution is reduced to substantially 0.5 part per mil-. lion or less.

8. The hereindescribed process for *the preparation of zinc sulfatesolutions for electrolysis by purifying the solutions from nicke copperand cadmium, which consists in adding a small proportion of acid,heating the solution to a temperature of 80 to 100 C., adding zincpowder, agitating the mixture whereby the nickel content of the solutionis reduced to substantially 0.5 part per million or less, and treatingthe precipitate with dilute acid to revivify the zinc for re-use.

In testimony whereof I aflix my signature.

SAMUEL FIELD.

